Thin film magnetic recording disks are widely used in the computer industry as a means for storing large amounts of digital data. Data are written onto and read from a rapidly rotating recording disk by means of a magnetic head transducer assembly that flies closely over the surface of the disk. With the development of increasingly small disk drives for incorporation into laptop and notebook size computers, as well as the need to continually increase the storage capacity of larger form factor disk drives, research is continuing to be done to optimize the magnetic performance of the disks. The magnetic disk which typically, according to present technology, is formed on an aluminum substrate comprises a series of alloys sputtered or deposited, one upon another, on the surface of the aluminum substrate. The thickness and characteristics of each of these layers is chosen to optimize the linear digital recording density of the magnetic recording disk. The objective is to form a recording disk on which the magnetic bits may be very closely spaced, without any nonuniformities occurring on the surface of the disk which could result in the inability to record data in that region. According to present technology, manufacture of the disk begins with an aluminum substrate. Because aluminum itself is relatively soft, a layer of nickel phosphorous is deposited over the aluminum to provide a hard interlayer which supports the remaining magnetic recording layers. On top of the nickel phosphorous, a layer of chromium (300-1500 .ANG.) is deposited. The actual magnetic recording layer which is a magnetic alloy comprising cobalt, chromium or nickel chromium is then sputtered onto a thickness of 200-400 .ANG.. Current technology has determined that the magnetic characteristics of the recording layer are optimized if a chromium interlayer of 30 to 100 .ANG. is then deposited, followed by a second magnetic recording alloy layer of about 200-300 .ANG..
Among the magnetic properties which are important to a high recording density and which will be referred to in the description below are:
1. Coercivity, defined as the magnetic field required to reduce the remanence magnetic flux to 0, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium allows adjacent recorded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density.
2. Magnetic remanence, which determines the signal amplitude which can be read from an isolated pulse stored in the medium--the greater the remanence, the greater the signal amplitude which can be detected in a reading operation.
3. Loop squareness, or the ratio of magnetic remanence to saturation moment. It can be appreciated that as the saturation field becomes smaller (approaches the coercivity), it takes less field strength to switch or "write to" the medium. In practical terms, this means that when a new signal is written over an old signal, the ratio of the old signal residual to the new signal is relatively small. This ratio is referred to as overwrite, a small overwrite ratio indicating good writability.
4. Bit shift or peak shift, a phenomenon which refers to the broadening between voltage peaks, as well as reduction in peak amplitude, which occurs in the read voltage waveform, where the peak-to-peak broadening time is typically less than about 25 nsec. It is desired to achieve low bit shifting, inasmuch as bit shifting limits the resolution at which adjacent peaks can be read, and thus places an upper limit on recording density.
5. Signal amplitude, or peak-to-peak amplitude of a single pulse, as a function of recording frequency. The recording density of the medium is related to the drop in signal amplitude at increasing recording frequency.
6. Signal resolution, defined as the ratio of the high-frequency track average amplitude divided by the low-frequency track amplitude. The recording frequency at which 70% resolution is achieved represents one measure of information storage density on the disk.